1. Field of the Invention
The present invention relates to speeding-up and image quality improvement in image processing and rendering processing for image formation.
2. Description of the Related Art
Processing regarding conventional image formation processing will be explained. First, an image forming apparatus receives PDL (Page Description Language) data. The PDL data is generated by an information processing apparatus generally called a personal computer (PC), and input to the image forming apparatus via a network or external storage medium. For example, the image forming apparatus can receive PDL data via a network interface, or acquire PDL data by connecting, to the USB interface of the image forming apparatus, a USB memory which saves the PDL data.
Then, a general-purpose processor in the image forming apparatus interprets the received PDL data. The general-purpose processor is a processor implemented by a von Neumann computer algorithm. The general-purpose processor can execute various arithmetic operations based on received instructions for received data.
Based on the interpretation result, the general-purpose processor or a RIP ASIC in the image forming apparatus executes rendering processing, obtaining raster image data. An image processing ASIC or the general-purpose processor in the image forming apparatus performs image processing (for example, image processing to print by the print engine of the apparatus) for the raster image data, obtaining raster image data having undergone the image processing. The raster image data having undergone the image processing is transferred to the print engine in the image forming apparatus. The print engine forms an image using the raster image data on a printing medium typified by paper.
The above-described rendering processing and image processing conventionally perform image compression processing to reduce the data amount to be saved in or loaded from a main memory, another storage device, or a medium, and increase the transfer speed.
The image compression processing adopts a lossy compression technique “JPEG (Joint Photographic Experts Group)” or the like. However, JPEG is accompanied by inverse DCT (Discrete Cosine Transform) and thus upsizes and complicates hardware and software.
As a method of decreasing the data amount without using inverse DCT (Discrete Cosine Transform) or the like, the data amount may be reduced by color-reduction processing (Japanese Patent Laid-Open No. 2008-271046). This method compresses the data amount by dividing image data into blocks each of 2×2 pixels, decreasing the number of colors of each divided block to two or less, and outputting color data of two colors and shape information about the layout of the two colors.
When PDL (Page Description Language) data is interpreted and rasterized into raster image data, the attribute (text, graphic (line image), or image (for example, photograph or pattern)) of each pixel is obtained. However, in Japanese Patent Laid-Open No. 2008-271046, the number of colors is decreased to two or less without taking account of the attribute of each pixel. When pixels with different attributes exist within one block, if color-reduction processing is performed without considering the attributes, pixels with different attributes may be changed to have the same color (same pixel value). If a portion (contour of a text or graphic) where contour information greatly affects the image quality is changed to have the same color as that of a background image, the contour becomes unclear, degrading the image quality.